The present invention relates to a gas sterilization apparatus and method wherein a heavier-than-air sterilizing gas or gas mixture introduced into the sterilizing chamber displaces air from the chamber.
The use of a heavier-than-air sterilizing gas or gas mixture, most commonly ethylene oxide or a mixture of ethylene oxide and some diluent such as a fluorocarbon gas, is well known in the art. In certain applications, a displacement method is used to replace the air in the sterilizing chamber with sterilizing gas. In this respect, the heavier gas is introduced into the chamber and simply allowed to displace the air upwardly and out of the chamber through some top located opening. Theoretically, when the gas is introduced into the chamber, the gas and air stratify with the heavier-than-air gas settling to the bottom of the chamber and displacing the air. Thereafter, as the depth of the gas increases, the air is forced up and out of the top located opening. The sterilizing period then begins when the articles to be sterilized are submerged in the gas.
In order to insure that the articles being sterilized are submerged in the gas, the usual procedure in the prior art is to simply calculate the quantity of gas required to fill the sterilizing chamber. This given quantity of gas is then introduced into the sterilizing chamber either on a weight or a time flow basis. Such prior art methods would be completely acceptable if the gas and air stratified into a lower layer of gas and an upper layer of air wherein the interface between the two layers was exactly defined or if the load space to air space is the same for each cycle. If such were the case, a calculated amount of sterilizing gas would replace a similar calculated amount of air so that the sterilizing chamber could be completely filled with the sterilizing gas and the air completely displaced.
As a practical matter, however, an intermediate layer forms between the sterilizing gas in the bottom of the sterilizer and the air in the top of the sterilizer. This intermediate layer is a mixture of air and the sterilizing gas. There are several factors which could lead to the formation of such an intermediate layer. For example, any turbulence created as the sterilizing gas enters the chamber will result in mixing of the gas and air. Also, any temperature differential between the articles being sterilized and the chamber may set up convection currents within the sterilizer which mixes the gas and the air. A third factor is termed "diffusion mixing" wherein the air and gas molecules tend to diffuse across the boundary between the upper layer of air and lower layer of gas. In any event, it is recognized that a relatively large and often variable intermediate layer, of a air-gas mixture forms between the layer of air and the lower layer of gas. Consequently, the prior art recognizes that a given volume of gas, no matter how carefully it is introduced into the sterilizing chamber, will not displace an equal amount of air.
To compensate for this, the prior art may employ some sort of gas monitor, analyzer or "presence of" detector to examine the exhaust of the chamber and indicate when there is sterilizing gas present in the air being displaced from the sterilizer. Depending upon the articles being sterilized, the introduction of gas can be stopped when sterilizing gas is just detected in the exhaust or when a particular concentration of sterilizing gas in the exhaust is reached. It should be readily appreciated that any such monitor, analyzer or presence of detector adds greatly to the cost of the sterilizing equipment.
A less expensive method to insure that articles being sterilized are completely submerged in the gas is to introduce an excess of gas into the sterilizing chamber. This method, however, is inherently inaccurate as the conditions which lead to the formation of the intermediate layer may vary from cycle to cycle and what may be an excess of sterilizing gas under one set of conditions may not be an excess under a slightly different set of conditions.
In the present invention, the sterilizing gas is introduced into the sterilizing chamber at a substantially constant flow rate. A low impedance restrictor is located in the exhaust to impede the passage of air and gas displaced from the chamber. Since the density of the sterilizing gas is greater than air, the gas exhibits different drag characteristics when flowing through the restrictor than does air, and it can be demonstrated that the flow rate of gas and displaced air leaving the sterilizing chamber through the restrictor is inversely proportional to the square of their densities. Because of these different drag characteristics, it has been experimentally verified that one equilibrium pressure level is established in the chamber during the time that only air is being displaced through the restricted exhaust and another higher pressure when only gas is passing through the restricted exhaust. When the intermediate layer, which is a mixture of gas and air, is displaced through the restrictor, the pressure in the chamber increases in direct proportion to an increase in gas concentration in the intermediate layer. Thus, at a constant input rate the pressure within the chamber can be directly correlated to the concentration of sterilizing gas within the chamber.